⚗️ Substances extractable with petroleum ether in water and wastewater — the gravimetric method

Physicochemistry PN-86/C-04573

Overview

The indicator „substancje ekstrahujące się eterem naftowym” [substances extractable with petroleum ether] is the sum of the organic substances which pass from a sample of water or wastewater into petroleum ether. The PKN catalogue card of the 1986 edition describes the method in a single sentence of principle: the extraction of organic substances with petroleum ether from an acidified and salted-out sample and the gravimetric determination of the content of extractable substances; the lower limit of determination is 0.5 mg/l. The standard itself has two pages and contains — according to the same card — general guidance, apparatus and instruments, reagents and solutions, preparation of the sample for testing, performance of the determination, calculation of the result and the final result of the determination. In the practice of a treatment plant this indicator is treated as a load of fats and oils: in a study of a municipal treatment plant with a flow of 26 000 m³/d the content of fatty and oily substances was determined by semi-micro extraction in a Soxhlet apparatus with petroleum ether, and the daily load in the raw wastewater averaged 2204.56 kg/d with an average removal efficiency of 42.7%.

The designation of the standard — because here a mistake is easily made. The PKN catalogue carries this sheet as **PN-C-04573-01:1986** (a hyphen before the number of the sheet, the year after a colon), date of publication 14.06.1986, technical body KT 122 for Water Quality – Chemical Tests – Organic Substances. The old-type designation **PN-86/C-04573-01** is the same standard, and it is precisely in that form that it appears in accreditation scopes (e.g. AB 1589: wastewater, substances extractable with petroleum ether, range (5.0 – 1200) mg/l, gravimetric method). The variant with a full stop ("PN-86/C-04573.01") does not exist in the PKN catalogue — it is an error of notation, not a separate document. MEASUREMENT OF THIS VARIANT IN THE SCOPES (23.09.2026), because "an error of notation" does not mean "a rarity": the core 04573 occurs in 34 entries at 28 laboratories, in five forms — "PN-86/C-04573/01" 18 times, "PN-86/C-04573.01" 9, "PN-86/C-04573-01" 5, and in addition two distorted records: "PN-86C-04573/01" without a hyphen after the year and "PN-C 04573-01:1986" with a space instead of a hyphen. The notation with a full stop is therefore the practice of roughly every fourth laboratory, not an exception. Contrary to what one might expect: the CATALOGUE designation ("PN-C-04573-01:1986", the new scheme with the year at the end) does not appear in the scopes EVEN ONCE — which is why the key of this entry is the old scheme "PN-86/C-04573", which covers all three sheet separators and gives 32 entries at 26 laboratories. The two distorted records remain outside the matching, because no prefix will cover them. Status: the card gives the date of withdrawal 18.01.2008 and has no "Superseded by" field, that is, the standard was withdrawn without being superseded. This does not prevent its use: in the Regulation of the Minister of Maritime Economy and Inland Navigation of 12.07.2019 (Journal of Laws 2019 item 1311), item 57 (the table of reference methodologies — a different table of the same regulation from item 55), "substancje ekstrahujące się eterem naftowym" [substances extractable with petroleum ether], has a dash in the column for the reference standard, and as the reference methodology "metodę specyficzną" [a specific method] is given, defined in a footnote as a multi-stage determination procedure, most often specific to the given indicator. The laboratory therefore itself names the document on which it based its procedure — and most often names this very withdrawn standard.

A caution about editions. The division of the range into two variants — 0.5÷4 mg/dm³ with extraction of the acidified sample in a liquid/liquid system and above 4 mg/dm³ with extraction in a Soxhlet apparatus of a sample acidified and evaporated to dryness — comes from the card of the **PREVIOUS edition, PN-C-04573-01:1978** (withdrawn on 26.08.1986 and superseded by the 1986 edition). The card of the 1986 edition, which is the one given in the field with the number of the standard, no longer repeats this division: it speaks only of extraction from an acidified and salted-out sample and of the lower limit of 0.5 mg/l. The description below keeps to this difference — everything that comes from the 1978 edition is marked as such. Nor should sheet 01 be confused with sheet 02 (PN-C-04573-02:1974, withdrawn on 13.12.2005), which concerns the determination of non-polar and polar organic substances extractable with petroleum ether in the range above 0.5 mg/dm³.

TAKEOVER OF THE ADDRESS AND CORRECTION OF THE DESIGNATION IN THE OTHER ENTRY (23.09.2026). This entry previously stood at the address "substancje-ekstrahujace-eterem-naftowym-woda-scieki-pn-c-04573-01", while the address "ekstrakt-eterowy" held an entry on THE SAME subject but with the WRONG standard: it gave "PN-C-04559:1982". **The PN-C-04559 series does not concern ether extract** — the PKN catalogue cards, read on 23.09.2026, list under it the determination of SUSPENDED SOLIDS: sheet 02 of 29-11-1972 (3 pages) is "Determination of total, mineral and volatile suspended solids by the gravimetric method", withdrawn on 01-08-2002 and superseded by PN-EN 872:2002, and sheet 03 of the same date (1 page) is "Determination of settleable suspended solids by the volumetric method", withdrawn on 16-01-2006. The year 1982 does not appear with this series either in the catalogue or in accreditation scopes.

WHAT THE TAB SHOWED FOR THAT ENTRY. The key "PN-C-04559" gave 3 entries at 3 laboratories — and these were laboratories determining SUSPENDED SOLIDS, shown to the reader as determining ether extract. This is worse than an empty tab and worse than a narrowed tab, because the list was neither empty nor incomplete — **it was true and the wrong one**. The answer came from the catalogue card, not from the measurement: the measurement showed three laboratories and had no way of saying that they measure something else.

The richer content therefore took over the older address, just as with PN-C-04584 a week earlier: the entry with the wrong standard and without a sources section (7987 characters, zero sources) gave way to the entry based on the PKN card (15 908 characters, seven sources), and the address at which readers found the page was preserved.

Method principle

The sample of water or wastewater is acidified and salted out, and the organic substances contained in it are transferred into petroleum ether. After the solvent has been separated and evaporated, the residue is weighed — hence the name "gravimetric method": the result is the mass of the dry residue of the extract converted to the volume of the sample. The method is by definition non-selective: what is determined is whatever is extracted by the given solvent (in the practice of a treatment plant treated as fats and oils), and not a particular chemical compound. The lower limit of determination given in the card of the 1986 edition is 0.5 mg/l. According to the card of the 1978 edition, at concentrations above 4 mg/dm³ the extraction was carried out in a Soxhlet apparatus on a sample acidified and evaporated to dryness, and below that — in a liquid/liquid system.

Applications

• Industrial wastewater discharged into water or into the ground — control of indicator no. 55 (the table of permissible values — a different table of the same regulation from item 57 with the reference methodologies) according to Journal of Laws 2019 item 1311 (50 mg/l for all kinds of wastewater, 20 mg/l for the sectors named in the annex)
• Industrial wastewater discharged into sewerage installations — control of the suppliers of wastewater by the operator of the network
• Balances of fats and oils in the wastewater and sludge lines of a treatment plant (raw wastewater, primary and surplus sludge, sludge liquors, treated wastewater)
• Testing of water and wastewater within the accreditation scopes of waterworks laboratories — e.g. AB 1589, wastewater, range (5.0 – 1200) mg/l, gravimetric method
• Assessment of the efficiency of grease traps and separators before wastewater is discharged into the sewerage

Key parameters

Standard

Standard number

PN-C-04573-01:1986

Title (PL)

Woda i ścieki — Badania zawartości substancji ekstrahujących się rozpuszczalnikami organicznymi — Oznaczanie całkowitej zawartości substancji organicznych ekstrahujących się eterem naftowym metodą wagową

Title (EN)

Water and wastewater — Tests for the content of substances extractable with organic solvents — Determination of the total content of organic substances extractable with petroleum ether by the gravimetric method

Step-by-step procedure

1. Taking and preparing the sample

   The sample of water or wastewater is taken and prepared in accordance with the clause „przygotowanie próbki do badań” [preparation of the sample for testing] of the standard. In the balances of a treatment plant the loads in the wastewater stream are determined from averaged 24-hour samples, and in the sludge stream — from several samples from each stream analysed.

2. Acidifying the sample

   The sample is acidified — the acidification converts the organic substances into a form extractable with petroleum ether.

3. Salting out

   The acidified sample is salted out. This step occurs in the 1986 edition; the card of the 1978 edition did not mention it.

4. Extraction with petroleum ether

   The organic substances are extracted with petroleum ether. The card of the 1986 edition gives only the extraction with petroleum ether from an acidified and salted-out sample and the gravimetric determination, without deciding in what system the extraction is carried out. The division into two variants we know only from the card of the 1978 edition: according to the card of the 1978 edition, at concentrations above 4 mg/dm³ extraction in a Soxhlet apparatus of a sample previously evaporated to dryness was used.

5. Separating the solvent layer

   A RECONSTRUCTION, not the content of the standard: the catalogue card gives only the extraction and the gravimetric determination, so we reconstruct the sequence of operations from the principle of the method. The ether layer is separated from the aqueous phase and transferred into a weighed vessel.

6. Evaporating the solvent

   The petroleum ether is evaporated without a naked flame and without sources of ignition — the flash point of the solvent is −40 °C.

7. Weighing the residue

   A RECONSTRUCTION, not the content of the standard. The dry residue is weighed on an analytical balance; it is that which is the result of the measurement.

8. Calculating the result

   The content of substances extractable with petroleum ether is calculated from the mass of the residue and the volume of the sample and is given in mg/l. The way of calculating and of giving the final result is described by the corresponding clauses of the standard.

9. Referring the result to the requirements

   For wastewater discharged into water or into the ground the permissible value is 50 mg/l for all kinds of wastewater, and 20 mg/l for the sectors named in the annex to the regulation Journal of Laws 2019 item 1311 (items 1, 2, 4, 5, 9 and 11).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Petroleum ether: H225 — highly flammable liquid and vapour; flash point −40 °C. Work only in a fume cupboard, without naked flames and sources of ignition, and evaporate without a flame
• H304 — may be fatal if swallowed and enters airways (aspiration hazard, category 1)
• H336 — may cause drowsiness or dizziness (action on the central nervous system when the vapours are inhaled)
• H315 — causes skin irritation: solvent-resistant gloves, protective goggles, laboratory coat
• H411 — toxic to aquatic life with long-lasting effects: the used solvent and the extracts must not be discharged into the sewerage
• Treat samples of raw wastewater as potentially infectious material — gloves, avoidance of aerosols

Frequently asked questions